Carbon dioxide (CO2) is currently considered as a major environmental pollutant that causes a dramatic increase in the global temperature or the so-called greenhouse effect. The contribution of CO2 to the climate warming is estimated to be about 66%. The CO2 level in atmosphere is now reported to be about 345 ppmv (parts per million by volume), and annually increases at a rate of about 1 ppmv due to human activities, especially in the case of using mineral fuel. Thus, the reduction of massive CO2 release into the atmosphere has attracted much attention of the scientists all over the world. In this regard, the use of CO2 as a polymerization monomer is of practical important. Aliphatic polycarbonates or the block copolymers of polycarbonate and polyether can be prepared via the direct copolymerization of CO2 with epoxides such as ethylene oxide (EO), propylene oxide (PO), isobutylene oxide (BO), and cyclohexene oxide (CHO). The copolymerization of carbon dioxide with epoxides to form poly(alkylene carbonate) polymers was first reported by Inoue and co-workers, Polymer Letters 7, 287(1969); Makromol. Chem. 130.210(1969); and described in U.S. Pat. No. 3,585,168. Other processes are described in U.S. Pat. Nos. 3,900,424; 3,953,383 and 5,026,676. However, the progress for the commercialization of these poly(alkylene carbonate)s that utilize this chemistry has been very slow, although there are numerous economic advantages associated with the use of an abundant, low cost material like carbon dioxide. The main reason lies with the practical difficulty in preparing large scale organometallic catalysts for commercial usage.
The catalysts reported by Inoue were prepared by reacting diethylzinc with compounds containing active protons, e.g., water, dicarboxylic acids, or dihydric phenols. Typical catalyst productivities ranged from 2.0 to 10.0 grams of polymer per gram of catalyst used, and most of the yields fall at the low end of this range. Long polymerization time periods of 24 to 48 hours were required in order to achieve satisfactory yields and higher molecular weights of the products. It should be noted that the inoue catalysts also generated noticeable amounts of byproducts of cyclic carbonate and polyether homopolymer that must be removed from the desired polycarbonate polymers.
Zinc carboxylates have also been described as effective catalysts for CO2 polymerization. Because zinc carboxylates are stable and safe compounds having no handling problem when comparing with diethylzinc, they are promising candidates for use as practical commercial catalyst systems. Soga and co-workers, Polymer J. 13(4), 407(1981) reported that the reaction products of zinc hydroxide and aliphatic dicarboxylic acids exhibited high activity for the copolymerization of carbon dioxide and propylene oxide. A variety of acids were tested, but only adipic and glutaric acid produced catalysts with higher activity than the known diethylzinc catalysts. Catalysts prepared from aromatic dicarboxylic acids were essentially inert under the polymerization conditions described by Soga.
Soga, Nippon Kagakkaishi 2, 295(1982) also reported another approach to improve the catalyst activity via supporting the catalyst on an inert Oxide Carrier. A supporting material can increase the surface area of active catalyst material, thereby enhancing the efficiency production of the aliphatic polycarbonate. In Soga's work, zinc acetate was selected as catalytically active component, and dissolved in some solvent, e.g. ethanol, to form a solution. After the silica support being added, the solvent is removed to give a supported catalyst. However, thus obtained catalyst has low activity due to the poor catalytic efficiency of zinc acetate. The supported catalysts of Soga are ineffective compared to the well-known diethylzinc based catalysts.
The metal salts of acetic acid are the third type of catalyst materials known to promote the copolymerization of CO2 with epoxides (Soga. et al., Makromol. Chem. 178, 893(1977)). Only zinc and cobalt can produce alternating copolymers from CO2 and epoxides, and the activity of these catalysts is lower than that derived from diethylzinc based catalysts.
In U.S. Pat. No. 4,783,445, Sun reported that soluble zinc catalysts can be prepared by reacting zinc oxide or zinc salts with a dicarboxylic acid anhydride or monoester in a suitable solvent such as the lower alcohols, ketones, esters and ethers. However, low catalytic activity is produced.
Among the catalyst systems reported in the literature up to that time, only zinc carboxylates based on adipic or glutaric acid seem potential for practical use on a commercial scale.